The userdata directory is for the user to put stuff. New files get downloaded there, along with user overrides being stored.
Files are copied to /addon/service.vpn.manager/UserDefined (I think) to be used. I can't exactly remember what triggers the copy but it's probably the validation of the connection. A bunch of other stuff gets updated/copied at the same time too.
You'll probably need to do your downloading, update the active connection, then restart it to take effect. There might be cases where it reverts any updates you've made as anything in the addon directory is fair game for updating with no easy way to disable this.
But VPN Mgr does what you're doing anyway for Nord - it's uses the API (providing the logon to the API works....) to go and fetch the recommended server during connection. It doesn't use static definitions like other providers.